Electron tube for magnetic induction accelerator



March 1953 SAMUEL T. YANAGISAWA ET AL 2,825,333

ELECTRON TUBE FOR MAGNETIC INDUCTION ACCELERATOR Filed June 3, 1953 2 Sheets-Sheet l WVENTOR HENRY F. MISERQCGHI SAMUEL TYANAGISAWA Ni TOR Y Mamh 1958 SAMUEL T. YANAGISAWA ET AL 3398259833 ELECTRON TUBE FOR MAGNETIC INDUCTION ACCELERATOR Filed June 3, 1953 2 Sheets-Sheet 2 Mega! INVENTOR HENRY F. WNSEROGCH! 56mm WE ATTORNEY SAMUEL T. (AN

United States Patent 9 Miserocc i, 01d Greenwich, Conn., assignors to Machlett Laboratories, Incorporated, Sprmgdale, Conn., a corporation of Connecticut Application June 3, 1953, Serial No. 359,296 9 Claims. (Cl. 313-62) This invention concerns an improved type of electron tube structure for use with magnetic induction accelerators, such as those commonly referred to as-betatrons. More specifically, this invention relates to means forimproving the functioning of the high-resistance coating which is applied to the internal surface of the hollow toroidal electron tube envelope and to means for protecting said coating.

The usual structure for this type of tube includes a hollow toroidal vacuum envelope having tubular horns or protrusions arranged to radially extend outwardly from the toroidal body. The end of one of the horns remote from the toroidal body is closed with a stern press which mounts the so-called electron injector and the anode structure. The injector consists of a cathode Within a cathode-potential focusing electrode oriented so that it will inject electrons tangentially into an essentially circular orbit, within and generally coaxial with the toroidal tube body. The electrons are caused to-follow such a circular orbit and to accelerate along said orbit under the influence of a changing magnetic field in which the lines of magnetic force are oriented generally parallel to the axis of revolution of the toroidal tube envelope body.

The cathode is essentially surrounded by the anode on three sides. Accordingly, the electrons must be injected into their equilibrium orbit through a slot in the anode. The anode structure is also provided with a target into which the accelerated electrons are deflected to produce X-rays. 7

Another horn on the toroidal envelope is closed by a wall which permits the passage of a grounding connection. The grounding connection is connected to -a continuous conductive coating of high resistance covering the entire internal surface of the hollow toroidal portion of the envelope.

The coating on the internal surface of toroidal elec-. tron tubes used in induction accelerators is used to conduct away stray electrons which accumulate on the envelope side walls. This coating, of necessity, must be of high resistance and of minimum thickness so that it will tend to support within its body a minimum of eddy currents and other currents induced by the changing magnetic field which might adversely aflect the particle acceleration and orbit determination function of the induction accelerator. The connection is made between the high-resistance coating and ground so that stray charges may be conducted away to ground. The high resistance of this coating, however, makes it difiicult for the stray electrons which it collects to be conducted away fast enough. Conduction is particularly difiicult where electrons must travel a long distance through the highresistance coating to the ground connection. This slow rate of conduction within thecoating itself may cause the charges collected on the walls of the electron tube to upset tube operation, often to a considerable degree.

The anode structure. of these tubes" is commonly connected to ground. In the'prior' art an anode ground connection has been accomplished by bringing a lead attached to the anode directly through the tubes vacuum envelope to ground. One result of this practice has been corona outside the vacuum envelope due to the high potential gradient between anode and cathode leads. Occasionally such high potential has caused high voltage puncture of the vacuum envelope, and the high gradient has been destructive of the high-resistance conductive coating on the internal surface of the envelope.

Sparking of a particularly serious nature may also occur in a tubular horn of the envelope at the edge of the high resistance coating because said edge forms a. sharp termination at which field gradients are extremely high. Destruction of the coating by sparking reduces the ability of the coating to conduct away stray particles that interfere with the elficiency of the induction accelerator.

The present invention provides means for correcting these problems by the use of a grounded highly conductive material in combination with a high-resistance coating. For instance, a highly conductive strip arranged generally parallel with the lines of magnetic force, may be placed almost all the way around the toroidal electron tubes inner wall. Use of a narrow strip of highly conductive material will provide a relatively well dis.- tributed collection means at the ends of many relatively short high-resistance paths from various points on the tube envelopes internal surface for electrons collected on the high-resistance material in all regions into which said strip extends. Once it receives these electrons, highly conductive material will quickly conduct them away to the grounding connection and ground.

Likewise, by the present invention, the anode and the grounding connection which leave the vacuum envelope through the other horn may be connected by such an internal highly conductive connection so that it is no longer necessary to bring out a separate anode lead through the same horn through which pass the cathode leads. Such an anode connection is particularly desirable where the anode lead would otherwise cause corona external of the envelope because of its proximity to a cathode lead.

In addition, by the present invention, it is also possible to place a highly conductive ring within the injector support horn over the edge of the high resistance coating. Such a ring may be composed of material better able to withstand the high gradient effects than is the coating. It may also present a more rounded edge so that the high gradient otherwise present at the sharp edge of the high resistance coating is diminished.

For a better understanding of our invention, reference is made to the following drawings:

Fig. 1 shows a plan view of the toroidal tube structure employing this invention;

Fig. 2 illustrates, in section, the portion of the toroidal tube structure of Fig. l which is adjacent to the horn protrusions;

Fig. 3 is an elevational view from the side of the in jector and anode assembly; and

Fig. 4 is an elevational view from the front of the injector and anode assembly toward the slot in the anode.

Referring to Fig. 1, it is seen that the body of this electron tube comprises a hollow toroidally-shaped vacuum envelope 10. This tube is designed to be located be tween pole pieces of induction accelerator magnets in such location that it is penetrated by a magnetic field having lines of force parallel to the tubes axis of revolution. The toroidal body portion of the vacuum envelope is advantageously composed of porcelain or other ceramic material. Protruding radially from the outside surface of the tube envelope 10 are two tubular horns 11 and 12, which open into and are part of the vacuum envelope. Horn member 11 has its end remote from the annular envelope body closed with a 13 which includes a stem press 14. Horn 12 is closed by reentrant glass wall member 15 which includes stem press 16. V

Referring to Fig. 2, the structures within the vacuum envelope supported by the horn members 11 and 12 may be seen. As may be seen in detailed views Figs. 3 and 4, 'ste'mpress 14 permits the passage through vacuum wall 13 of cathode leads 17. These leads are advantageously caused to penetrate the vacuum wall in essentially parallel radial paths into the toroidal envelope portion; Since these leads are relatively rigid, they support the cathode 18. The cathode 18 is preferably composed of a helix reentrant glass wall member of emissive material such as tungsten, thoriated tungsten,

or the like.- Cathode 18 is usually connected with its axis generally perpendicular to the leads 17.

- The anode structure 20 consists of a U-shaped metallic member, the open end of which is toward the horn 11 and the parallel sides of which are made to extend generally radially into the vacuum envelope, essentially parallel to magnetic lines of force. The anode 20 is made to enclose the cathode structure on three sides. A small slot-like opening 21 is formed in one wall of the U-shaped anode. Through opening 21 a beam of electrons emitted from the cathode 18 is injected tangentially into the circular orbit wherein inductive acceleration takes place. The anode may be supported upon tubular glass member 22 by means of support members 23.

Within the anode, surrounding the cathode and at cathode potential (by virtue of connection to one of the cathodesupports) is a U-shaped focusing member 25.

The legs of this U-shaped focusing member are generally perpendicular to the legs of the U-shaped anode 20. The U-shaped focusing member is oriented so that its open end is aligned with slot opening 21 in anode 20.

The usual expedient for applying potential to the anode in the prior art has been by means of lead 27, here shown dotted. Lead 27 was brought through the tubular portion of wall member 13. Lead 27 was then attached to the anode. It is common to keep the anode at ground potential, and, accordingly, lead 27 was grounded externally of the vacuum envelope, as by connection to a grounding lead.

Within the vacuum envelope a continuous coating 28 of high-resistance conductive material is applied to cover the entire internal surface of the annular portion of the vacuum envelope. The coating extends into the horns 11 and 12 beyond the magnetic field where said coating is terminated. This coating is made thin and of high resistance so that it will permit a minimum of spurious currents to be induced therein. The function of this coating, which may be advantageously composed of palladium or platinum, is to conduct away stray electrons which may strike the envelope walls. Accordingly, it is desirable that the internal coating be grounded in some manner.

The second born 12 permits penetration of the tube envelope by a lead connected to the highresistance coating. This lead 29 is connected in some manner to the internal. high-resistance coating and then passes through the stem press 16 to a grounding connection. A simple connection between the grounding lead 29 and the highresistance coating is accomplished by employing tubular metallic member 30. Member 30 has spring fingers thereon, which spring fingers contact the high-resistance coating. 'Lead 29 is then connected to tubular member 30 instead of directly to the grounding coating.

'The, present invention provides a low-resistance strip 32 of conductive material which is advantageously made to extend along the inner surface of the outside wall of the hollow toroidal body within the vacuum envelope. This conductive strip may be made to extend almost all the way around the toroidal envelope in contact all the way, or at frequent intervals, with the high-resistance coating. However, the-conductive strip should never form a closed circuit or ring because currents would be induced by the magnetic field in such a circuit. Such an extent of conductor provides a low-resistance path along which electrons collected by the high-resistance coating and transferred to the conductive stri may easily flow to ground. Such alow-resistance (or highly conductive) strip effectively reduces the mean distance which electrons have to travel through the high-resistance coating. Thus, in effect, it speeds up the flow of such electrons to ground and prevents disruption of the functioning of the tube bya build-up on the high resistance coating of charges which might interfere with the magnetic field. The low-resistance strip 32 is in turn terminated in born 12 where it is connected by tubular member 30, which in turn is connected to ground connection lead 29.

The strip 32 is advantageously made quite narrow and arranged so that it is everywhere generally parallel to lines of magnetic flux. The strip is advantageously composed of silver, or other material, painted on to the internal wall of the vacuum envelope. Various conductive paints well known in the art are available for the purpose. Various application processes are possible. One process of application consists of placing a quantity of fluid conductive paint within the toroidal body through a horn while-the axis of revolution of the body is held parallel to the ground and the horn is also essentially parallel to the ground, and then slowly rotating the toroidal body about its axis. Such a strip will be in good contact with the high resistance coating and will not be subject to the objection that it extends out into the vacuum envelope away from the wall where it will interfere with the electrons being accelerated. However, it is conceivable to use materials such as copper strip, or other highly conductive strip material, which is placed in the envelope in such manner that it will contact with the conductive coating on the envelope side walls at frequent intervals and remain generally parallel to the lines of magnetic force.

tain the anode at ground potential while maintaining the cathode at a high negative potential. It has been difiicult in the prior art to bring the anode and cathode leads out of the vacuum envelope through the same horn member. Attempts to do this, using a lead 27, previously alluded to, have resulted in corona around the cathode leads and occasional puncture of the tube envelope due to high voltage gradients.

By our invention an internal electrical connection is made between the anode and ground connection. We prefer to employ portion 32a of highly conductive painted silver strip 32 between a lead 33 connected to the anode and the tubular ground connection member 30. Any other strip type or internal conductive connection may be employed as long as its surface is kept essentially parallel to lines of magnetic force, and it is kept away from the path in which electrons are normally accelerated With the anode thus internally connected to a member at ground potential, it is unnecessary to bring the anode lead out of the same horn member as the cathode leads so that the opportunity for corona to occur is eliminated.

The edge of the high-resistance coating 28, which terminates within horn member 11, represents a very sharp conductive edge at which the potential gradient may become quite severe. Thus, even with the anode lead eliminated, it is still possible for sparking and arcing and other destructive effects to occur between this edge and the cathode leads. Accordingly, by our invention, we propose to eliminate this sparking and other destructive effects by eliminating the sharp edge of the conductive material. We do this by interposing between the cathode leads and the high-resistance coating a ring 34 of highly conductive material which is better able to withstand the gradient than is the high-resistance coating. If the ring thus interposed is made thick enough to have a relatively large We have also discovered an advantageous way to mainradius of curvature at its edges, the accompanying reduction in gradient also reduces the opportunity for breakdown. The high-resistance conductive coating 28 should extend well out into the horn out of the magnetic field.

Thus, the conductive ring at its end, which ring is by its very arrangement largely not parallel to axial flux lines, will not be subject to damage due to heavy induced currents. We prefer to form this ring by painting it onto the high resistance coating material with silver paint. However, other conductive paint or strip or tubular stock conductive material may be employed. Such material may have the advantage of reducing the gradient at the ends of the ring because they may be made thicker than the painted silver ring. Such materials, however, are more difiicult to place in good contact with the edge of the high-resistance material in the horn.

It has been found that the grounding lead 29 tends to break off outside the vacuum enveloped adjacent the point at which it penetrates said envelope. Accordingly, lead 29 is reinforced after passing through stem press 16 and out of the reentrant portion of glass wall member 15. A disc 35 is placed across the end of the reentrant portion of wall and mounted in place as by using one of the basing adhesives available for the purpose. The lead 29 is brought axially through this disc 35 and may be fixed to the disc, if preferred. In this manner, bending stresses and strains are imposed upon disc member 35 rather than upon the relatively fragile glass stem press 16.

In operation, the cathode thermally emits electrons and, under the influence of the focusing member and the anode 20, said electrons are injected essentially tangentially into their circular orbit. The magnetic flux lines cutting the toroidal body of the tube then act upon these electrons to direct them into a circular orbit and to accelerate them. The electrons are generally confined to an equilibrium orbit of a specfic radius by a suitable time varying magnetic field. In the structure as described heretofore this orbit is somewhat smaller in radius than an orbit which would be intercepted by the injector and anode structure. When the electrons have been accelerated to some desired energy level, such as 24 MEV, their orbit is expanded until they are caused to strike a small target 36 atop the anode structure, thereby producing high energy, highly penetrating X-rays. In the course of acceleration those electrons which go astray from the induced path are captured or collected on the highresistance conductive coating 28 on the inner wall of the hollow toroidal envelope. These electrons are conducted thence to the highly conductive strip and then rapidly to ground.

The preferred forms and embodiments of our invention have been described in some detail. However, it will be obvious to one skilled in the art that many variations of our invention are possible. Any of the various features of this invention, individually or in combination with one another, may be employed in an electron tube structure for use in an induction accelerator without employing other features of the invention.

We claim:

1. An electron tube structure for use as an integral part of a magnetic induction accelerator, comprising a hollow toroidally shaped vacuum envelope, a high resistance conductive coating on the internal surface of the vacuum envelope, cathode and anode electrodes within the vacuum envelope, one of which electrodes is connected to a first conductor which penetrates the vacuum envelope in the region adjacent said electrode, a second conductor penetrating the vacuum envelope remote from the point of penetration of the first conductor, and a low resistance connecting conductor within the envelope extending between the other electrode and said second conductor and lying in the general direction of the magnetic lines of force therein.

2. The structure as described in claim 1 in which the low resistance connecting conductor between the electrode and the second conductor is a strip of highly conductive material supported on the side wall of the vacuum envelope.

3. The structure as described in claim 1 in which the second conductor is connected to the anode electrode and the low resistance connecting conductor is a strip mounted on a side wall of the hollow toroidally shaped envelope.

4. An electron tube structure for use as an integral part of a magnetic induction accelerator, comprising a hollow toroidally shaped vacuum envelope having a pair of radially directed tubular horns closed vacuum tight at the end remote from the toroidal portion of the envelope, a high resistance conductive coating on the internal surface of the vacuum envelope, and electrode structures within the vacuum envelope including anode and cathode elements extending into the toroidal portion from one of the horns, cathode leads penetrating the vacuum envelope through said horn, a grounding electrical connection for the high resistance coating including a conductor penetrating the vacuum envelope through the other horn and a low resistance connecting conductor within the envelope extending between the anode and the grounding connection and lying in the general direction of lines of force therein.

5. The structure as described in claim 4 in which the low resistance connecting conductor between the electrode and the grounding connection is a strip of highly conductive material carried by a side wall of the vacuum envelope.

6. An electron tube for use as an integral part of a magnetic induction accelerator comprising a toroidal evacuated envelope having a tubular neck portion extending away from the envelope and having a vacuum wall at the end thereof remote from the toroidal portion, anode and cathode electrodes within the vacuum envelope, a conductor connected to the cathode and passing through the neck portion, a high resistance conductive coating on the internal surface of said envelope, which conductive coating extends into the neck portion of the envelope and is there terminated, and a ring of highly conductive material interposed between the conductor passing through the neck portion and the edge of the high resistance coating so that said highly conductive material extends beyond the high resistance conductive coating in close proximity to the envelope sidewalls.

7. The structure as described in claim 6 in which the ring of highly conductive material is in intimate contact with the edge of the high resistance coating.

8. An electron tube for use as an integral part of a magnetic induction accelerator which comprises a toroidal vacuum envelope, a high resistance coating on the internal surface of said toroidal envelope, a highly conductive strip which extends around a major portion of said envelope, said highly conductive strip being arranged along a wall of the vacuum envelope so that it presents a surface which is parallel to the path of the lines of magnetic force within the tube and a grounding lead which penetrates the vacuum envelope and is connected to the conductive strip.

9. The structure as described in claim 8 in which the ,highly conductive strip is in intimate contact with the high resistance coating.

References Cited in the tile of this patent UNITED STATES PATENTS 2,193,602 Penney Mar. 12, 1940 2,510,448 Wideroe June 6, 1950 

